Most often in May, 29 days.
Highest follower count seen that year: 158,635.
January
Jan 4 07:00–18:00 UTC Jan 7 07:00–17:00 UTC Jan 10 07:00–17:00 UTC Jan 11 07:00–17:00 UTC Jan 12 07:00–17:00 UTC Jan 13 07:00–17:00 UTC Jan 14 07:00–18:00 UTC Jan 15 07:00–17:00 UTC Jan 16 07:00–17:00 UTC Jan 18 07:00–17:00 UTC Jan 19 07:00–17:00 UTC Jan 21 07:00–18:00 UTC Jan 22 09:00–16:00 UTC Jan 23 07:00–18:00 UTC Jan 28 08:00–18:00 UTC Jan 29 08:00–18:00 UTC Jan 31 07:00–17:00 UTC
February
Feb 1 07:00–09:00 UTC Feb 10 20:00–24:00 UTC Feb 11 00:00–06:00, 19:00–23:00 UTC Feb 12 00:00–06:00, 19:00–24:00 UTC Feb 13 01:00–05:00, 19:00–23:00 UTC Feb 14 00:00–02:00 UTC Feb 15 19:00–23:00 UTC Feb 16 00:00–06:00, 19:00–24:00 UTC Feb 17 01:00–06:00 UTC Feb 19 20:00–24:00 UTC Feb 20 00:00–06:00 UTC Feb 21 19:00–23:00 UTC Feb 22 00:00–06:00, 19:00–24:00 UTC Feb 23 00:00–06:00, 19:00–24:00 UTC Feb 24 01:00–06:00 UTC Feb 25 08:00–16:00, 20:00–24:00 UTC Feb 26 01:00–17:00 UTC Feb 27 07:00–18:00, 20:00–24:00 UTC Feb 28 00:00–06:00, 08:00–17:00, 19:00–24:00 UTC
March
Mar 1 00:00–06:00, 19:00–24:00 UTC Mar 2 00:00–05:00, 19:00–23:00 UTC Mar 3 00:00–06:00 UTC Mar 5 07:00–17:00 UTC Mar 6 08:00–17:00, 19:00–24:00 UTC Mar 7 01:00–06:00, 08:00–17:00, 19:00–24:00 UTC Mar 8 00:00–06:00, 19:00–24:00 UTC Mar 9 01:00–06:00, 20:00–24:00 UTC Mar 10 00:00–18:00 UTC Mar 11 08:00–16:00, 19:00–24:00 UTC Mar 12 00:00–06:00 UTC Mar 14 20:00–24:00 UTC Mar 15 01:00–06:00, 08:00–09:00, 19:00–24:00 UTC Mar 16 01:00–17:00, 20:00–24:00 UTC Mar 17 00:00–06:00 UTC Mar 18 07:00–23:00 UTC Mar 19 00:00–06:00 UTC Mar 20 07:00–17:00, 19:00–23:00 UTC Mar 21 19:00–23:00 UTC Mar 22 00:00–04:00, 19:00–23:00 UTC Mar 23 00:00–06:00, 20:00–23:00 UTC Mar 24 00:00–06:00 UTC Mar 25 19:00–24:00 UTC Mar 26 00:00–02:00, 20:00–23:00 UTC Mar 27 00:00–06:00, 08:00–18:00 UTC Mar 28 09:00–23:00 UTC Mar 29 00:00–06:00, 08:00–23:00 UTC Mar 30 00:00–06:00, 19:00–23:00 UTC Mar 31 00:00–06:00, 08:00–23:00 UTC
April
Apr 1 01:00–06:00, 08:00–18:00 UTC Apr 3 08:00–17:00 UTC Apr 4 08:00–23:00 UTC Apr 5 00:00–06:00, 20:00–23:00 UTC Apr 6 09:00–19:00 UTC Apr 7 08:00–19:00 UTC Apr 8 19:00–23:00 UTC Apr 9 00:00–06:00, 20:00–23:00 UTC Apr 10 01:00–06:00, 16:00–22:00 UTC Apr 11 09:00–23:00 UTC Apr 12 00:00–06:00, 19:00–23:00 UTC Apr 13 00:00–06:00 UTC Apr 15 09:00–24:00 UTC Apr 16 00:00–06:00, 08:00–23:00 UTC Apr 17 01:00–07:00, 10:00–19:00 UTC Apr 18 09:00–24:00 UTC Apr 19 02:00–18:00, 20:00–24:00 UTC Apr 20 00:00–07:00, 20:00–23:00 UTC Apr 21 03:00–06:00 UTC Apr 22 08:00–19:00, 22:00–24:00 UTC Apr 23 00:00–04:00, 19:00–23:00 UTC Apr 24 00:00–07:00, 09:00–24:00 UTC Apr 25 01:00–07:00, 12:00–24:00 UTC Apr 26 01:00–06:00, 11:00–19:00 UTC Apr 27 09:00–20:00 UTC Apr 28 11:00–19:00 UTC Apr 29 07:00–17:00, 19:00–24:00 UTC Apr 30 00:00–17:00 UTC
May
May 1 08:00–19:00 UTC May 2 09:00–12:00, 20:00–23:00 UTC May 3 00:00–06:00, 10:00–23:00 UTC May 4 01:00–06:00, 10:00–20:00 UTC May 5 09:00–16:00 UTC May 6 19:00–23:00 UTC May 7 00:00–06:00, 20:00–23:00 UTC May 8 01:00–17:00, 19:00–24:00 UTC May 9 02:00–06:00, 10:00–24:00 UTC May 10 01:00–06:00, 19:00–23:00 UTC May 11 00:00–07:00, 21:00–24:00 UTC May 12 00:00–06:00 UTC May 13 07:00–16:00 UTC May 14 07:00–16:00 UTC May 15 08:00–17:00, 20:00–24:00 UTC May 16 01:00–18:00, 20:00–24:00 UTC May 17 00:00–18:00, 20:00–24:00 UTC May 18 01:00–06:00, 08:00–17:00 UTC May 19 08:00–18:00 UTC May 21 20:00–24:00 UTC May 22 00:00–06:00, 08:00–18:00, 20:00–23:00 UTC May 23 00:00–12:00, 15:00–19:00, 21:00–24:00 UTC May 24 01:00–06:00, 19:00–23:00 UTC May 25 00:00–05:00, 09:00–19:00 UTC May 27 19:00–23:00 UTC May 28 00:00–16:00, 20:00–23:00 UTC May 29 00:00–16:00, 20:00–24:00 UTC May 30 01:00–16:00 UTC May 31 07:00–16:00, 20:00–23:00 UTC
June
Jun 1 00:00–06:00 UTC Jun 2 07:00–16:00 UTC Jun 3 07:00–16:00, 20:00–24:00 UTC Jun 4 01:00–06:00 UTC Jun 5 21:00–24:00 UTC Jun 6 01:00–07:00, 20:00–23:00 UTC Jun 7 02:00–07:00, 20:00–23:00 UTC Jun 8 01:00–06:00 UTC Jun 9 09:00–11:00 UTC Jun 10 08:00–17:00 UTC Jun 11 07:00–16:00, 20:00–24:00 UTC Jun 12 01:00–16:00, 20:00–23:00 UTC Jun 13 00:00–16:00 UTC Jun 15 01:00–06:00 UTC Jun 17 07:00–17:00 UTC Jun 18 07:00–16:00 UTC Jun 19 07:00–16:00 UTC Jun 20 19:00–23:00 UTC Jun 21 00:00–05:00, 07:00–17:00, 20:00–23:00 UTC Jun 22 01:00–23:00 UTC Jun 23 04:00–06:00, 20:00–24:00 UTC Jun 24 01:00–06:00, 20:00–23:00 UTC Jun 25 00:00–06:00, 21:00–24:00 UTC Jun 26 01:00–05:00 UTC Jun 28 07:00–17:00, 20:00–24:00 UTC Jun 29 01:00–06:00, 08:00–16:00, 19:00–24:00 UTC Jun 30 00:00–18:00, 20:00–24:00 UTC
July
Jul 1 01:00–06:00 UTC Jul 2 07:00–16:00, 19:00–23:00 UTC Jul 3 00:00–06:00 UTC Jul 4 07:00–11:00, 13:00–18:00 UTC Jul 5 08:00–18:00, 20:00–23:00 UTC Jul 6 00:00–02:00, 10:00–19:00 UTC Jul 8 10:00–19:00, 21:00–24:00 UTC Jul 9 01:00–02:00, 10:00–19:00 UTC Jul 12 10:00–19:00 UTC Jul 13 10:00–19:00 UTC Jul 14 22:00–24:00 UTC Jul 15 00:00–07:00, 21:00–24:00 UTC Jul 16 05:00–06:00, 10:00–19:00, 21:00–24:00 UTC Jul 18 10:00–19:00 UTC Jul 19 09:00–18:00, 21:00–24:00 UTC Jul 20 01:00–05:00, 10:00–19:00 UTC Jul 22 10:00–19:00 UTC Jul 23 10:00–18:00 UTC Jul 24 10:00–18:00, 22:00–23:00 UTC Jul 26 02:00–06:00, 21:00–24:00 UTC Jul 27 00:00–02:00 UTC